Heretofore, as a working fluid such as a refrigerant for a refrigerator, a refrigerant for an air-conditioning apparatus, a working fluid for power generation system (such as exhaust heat recovery power generation), a working fluid for a latent heat transport apparatus (such as a heat pipe) or a secondary cooling fluid, a hydrofluorocarbon (HFC) such as difluoromethane (HFC-32), tetrafluoroethane or pentafluoroethane (HFC-125), which has less influence over the ozone layer, has been used. Further, by new environmental regulation, a new refrigerant to be used for a refrigerator, an air-conditioning apparatus, a heat pump apparatus, etc. is required, and compounds having low global warming potential (GWP) attract attention. In this specification, abbreviated names of halogenated hydrocarbon compounds are described in brackets after the compound names, and in this specification, the abbreviated names are employed instead of the compound names as the case requires. Further, (E), (Z) or the like before the compound names or after the abbreviated names of compounds represent E-form or Z-form of geometric isomers.
As such a working fluid, a hydrofluoroolefin (HFO) having a carbon-carbon double bond is proposed, which has less influence over the ozone layer and which has less influence over global warming, since the carbon-carbon double bond is likely to be decomposed by OH radicals in the air.
Particularly, as the HFO, trifluoroethylene (HFO-1123) is considered as promising as an alternative to a saturated hydrofluorocarbon (HFC) refrigerant having a high GWP.
When a HFO is used as a refrigerant, if the HFO contains a large quantity of water (moisture), various problems in the reliability and in the performance may arise. In order to suppress such unfavorable influences, the water content is preferably as low as possible.
As a method for drying the HFO, Patent Document 1 discloses a method of bringing a gas containing a fluoropropene into contact with a molecular sieve having a maximum opening of from about 3 Å to about 5 Å. It discloses as the molecular sieve zeolites 3A, 4A and 5A, and discloses an example of drying (desiccating) a fluid containing 2,3,3,3-tetrafluoropropene (HFO-1234yf) using such a molecular sieve. Further, Patent Document 2 discloses a method for removing water in a fluid containing a fluoroolefin and a C1 alkane by bringing the fluid into contact with synthetic zeolite 3A.
However, a method for drying a fluid containing HFO-1123 is not disclosed in Patent Documents 1 and 2. Particularly in a case where the fluid contains HFO-1123 and a C1-5 alkane or alkene in which at least one hydrogen atom may be substituted with a chlorine atom or a fluorine atom, it is found that by the method disclosed in Patent Document 1, the C1-5 alkane or alkene is adsorbed in the molecular sieve and removed in some cases, and it is not possible to remove only water in the fluid.
Further, in a method for producing HFO-1123, HFO-1123 is obtained as a compound contained in a fluid containing a C1-5 alkane or alkene in which at least one hydrogen atom may be substituted with a chlorine atom or a fluorine atom, and it is difficult to obtain high purity HFO-1123 from the fluid with a sufficiently high yield. For example, in a case where HFO-1123 is to be obtained by subjecting chlorotrifluoroethylene (CTFE) to hydrogen reduction in the presence of a palladium or platinum catalyst, hydrogen reduction excessively proceeds since a transition metal catalyst is used, and together with the desired HFO-1123, excessively reduced (E)-1,2-difluoroethylene (HFO-1132(E)) forms as a by-product. It is difficult to purify and separate HFO-1123 since HFO-1132(E) has a boiling point close to that of the desired HFO-1123, and high purity HFO-1123 cannot be obtained with a sufficiently high yield.